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"..This is because most image files store data from top to bottom, while the software utilizes traditional Cartesian coordinates with the origin in the conventional lower-left corner. If your raster appears upside-down, you may need to add a minus sign. The parameter therefore describes the map distance between consecutive image lines."

So you are right, if the origin is at top-left the distance between consecutive image line is positive and hence positive Y values.

Mmmm... still a doubt. I know cartesian coordinate system sets the origin in the bottom left corner, while images do it in the top left corner. So, in georeferenced images, you need a negative Y resolution, otherwise you get a south up image. Ok so far. But, for example, why doesn't gdalbuildvrt accept positive y resolutions? If this doesn't mean a mistake, just a flipped image, what's the problem?
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jorgeas80Aug 10 '12 at 20:32